Why modern submarines are costly commerce raiders

Are modern submarines useful in an economic-warfare role against merchant shipping?

Not especially; their limited torpedo load makes individual merchant sinkings expensive.

Contemporary submarines do not carry enough weapons for an economical campaign against merchant traffic. A torpedo exploding along a merchant ship's keel can break its back and leave a long, expensive repair even if the hull remains afloat. Each attack still consumes a scarce and costly weapon, and the submarine's limited magazine would be exhausted quickly if used one ship at a time. Commerce raiding is technically possible but a poor fit for an expensive submarine with other high-value naval targets and missions.